In the present, the wide angle projection lens can effectively shorten the distance between the projection screen and the projector, and obtain a large size of the projection image. However, an aberration derived from the wide angle projection lens, such as distortion, field curvature, astigmatism, etc., are all the problems that must be faced when designing the wide angle projection lens. In order to ensure the optical quality, the wide angle lens usually narrows the numerical aperture, and the f-numbers of the wide angle lens are mostly ranged between 2.4˜2.7, or even larger.
In general, the wide angle projection lens, which can shorten the focal length and overcome the problems of the aberration at the same time, are usually designed in three manners:
1. Refraction type: the wide angle lens is designed with the whole refractive lenses. The spherical lenses or the aspherical lenses are usually used in the wide angle lens to achieve the function of wide angle.
2. Reflection type: the wide angle lens is designed with the whole reflective minors. The spherical, the aspherical or plane reflectors are usually used in the wide angle lens.
3. Hybrid type (refraction+reflection): the lenses will be used at the object side and at last, the reflector will be used at the image side to achieve the wide angle function. The reflector can also be classified into three typical types, which are the plane reflector, the concave reflector and the convex reflector respectively.
However, the above-mentioned three manners remains having disadvantages and problems as follows:
1. There are too many refractive lenses in the refraction type wide angle lens, which causes the lens barrel being too long and dispersed, such as lateral color, axial color, aberration and geometric distortion, produced by the refraction type wide angle lens are not easy to be eliminated. In addition, the large numbers of the lenses in the refraction type wide angle lens make a high complexity when designing, and the manufacturability thereof is very low.
2. Since there is none of refractive lens in the reflection type wide angle lens, which can achieve the zero-dispersion, the reflectors designed to be used in the reflection type wide angle lens require many aspherical reflectors or free-surface reflectors, which causing the manufacturability thereof is very low.
3. The hybrid type (refraction+reflection) mixes the refraction type and reflection type, which can balance the optical properties and manufacturability at the same time, and is the common type on the market currently. In order to achieve the ultra-wide angle and avoid the optical path length larger than the projection distance, the straight optical path must be turned, so that the optical components and machinery dimensions also become larger, resulting in a substantial increase for the difficulty of the component manufacturing. If using the complicated free-surface reflector, the assembly tolerances may also quite sensitive. In addition, in the three above-mentioned reflectors, the plane reflector has no curvature, so that the lens must rely on the adjustment of the distance between the lens and the reflector to achieve a wider angle. That is why the lens becomes longer which sacrifices the space. Although the convex reflector has curvature which can compensate for the aberration, the convex reflector will be difficult to be covered by this mechanism owing to the geometry form. Therefore, the lens mostly belongs to the exposed design in the present. However, the exposed design will increase the probability of the damage of the mirror surface, causing that there is not any dustproof effect in this exposed design and thus the stability thereof is not well, which only suitable for wall-mounted design. The concave reflector has the function of the convex reflector and can overcome the problem of the exposed mirror, which is a better option for the designer to design the wide angle projection lens. Although the concave reflector collocate with the hybrid design can resolve the problems caused by the plane and the convex reflectors, the volume is larger, comparing to the general front projector, for achieving the wide angle effect. For shortening the length of the lens barrel, the reflector will be added into the lens for turning. However, the width or the height of the lens will be increased when adding the reflector into the lens. Such kind of design cannot shorten the lens barrel effectively.
Accordingly, to design a wide angle projection lens with shortened focal length for achieving the decreased of the lens barrel and will not produce the aberration derived in general wide angle projection lens under the condition of a large numerical aperture, is a difficult challenge for overcoming.
It is therefore attempted by the applicant to deal with the above situation encountered in the prior art.